This bottom-quintile to top-quintile measurement is annoying. A 'low mobility' area could be largely due to less upwardly-mobile poor or less downwardly-mobile

This bottom-quintile to top-quintile measurement is annoying. A 'low mobility' area could be largely due to less upwardly-mobile poor or less downwardly-mobile affluent, and thanks to the researchers' weak choice of metric it's impossible to tell which.